Caption: Human heart cells on a ECG (electrocardiogram) chip, coloured light micrograph. These cells are derived from human embryonic stem cells (HESCs), which have been triggered to develop into cardiac muscle cells. Unlike skeletal muscle, cardiac cells do not need any external stimuli to contract rhythmically, and so beat even in culture. The ECG chip is measuring the electrical activity (the beating) of the cells. Stem cells could be used to treat heart disease. Blocked arteries can prevent oxygen reaching parts of the heart, killing cardiac muscle tissue. Although blood flow can be restored, as yet the cells cannot. Potentially, stem cells could be injected into the heart to replace the dead tissue.